A tilt rotor or tilt wing aircraft typically employs a pair of rotor systems which are supported at the outermost end of a wing structure and are pivotable such that the rotors thereof may assume a vertical or horizontal orientation. In a horizontal orientation, the aircraft is capable of hovering flight, while in a vertical orientation, the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft.
Currently, tilt rotor/tilt wing aircraft employ conventional fixed-diameter rotor systems which, in the aerodynamic and aeroelastic design thereof, attempt to blend the competing requirements of hovering and forward flight modes of operation. For example, with regard to hovering flight, it is generally advantageous to employ a large diameter rotor to improve hovering performance by lowering disk loading, reducing noise levels, and reducing downwash velocities. Conversely, a relatively small diameter rotor is desirable in forward flight to improve propulsive efficiency by minimizing blade aero-elastic properties, minimizing blade area, and reducing tip speed (Mach number).
Variable Diameter Rotor (VDR) systems are known to provide distinct advantages over conventional fixed-diameter rotors insofar as such systems are capable of successfully operating in both modes of operation. That is, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency.
An example of a VDR system and VDR blade assembly therefor is shown in Fradenburgh U.S. Pat. No. 3,768,923 wherein each blade assembly includes an outer blade segment which telescopes over a torque tube member so as increase or decrease the rotor diameter. The outer blade segment includes a structural spar, i.e., the foremost structural element which carries the primary loads of the outer blade segment, a leading edge sheath assembly and trailing edge pocket assembly, which sheath and pocket assemblies envelop the spar section to define the requisite aerodynamic blade contour. The torque tube member mounts to a rotor hub assembly and receives the spar member of the outer blade segment. The torque tube member, furthermore, functions to transfer flapwise and edgewise bending loads to and from the rotor hub assembly while furthermore imparting pitch motion to the outer blade segment. The resultant torque tube/spar assembly forms a central channel for housing a retraction/extension mechanism. The retraction/extension mechanism includes a threaded jackscrew which may be driven in either direction by a bevel gear arrangement disposed internally of the rotor hub assembly. The jackscrew, furthermore, engages a plurality of stacked nuts which are rotationally fixed by the internal geometry of the torque tube member yet are permitted to translate axially along the jackscrew upon rotation thereof. Furthermore, centrifugal straps extend from each nut and are affixed via a retention plate to the tip end of the spar member. As the jackscrew turns, the stacked nuts are caused to translate inwardly or outwardly, thereby effecting axial translation of the outer blade segment. Systems relating to and/or further describing VDR systems are discussed in U.S. Pat. Nos. 3,884, 594, 4,074,952, 4,007,997, 5,253,979, and 5,299,912.
While the operational requirements, i.e., the imposed loads, motions, vibratory environment etc., of conventional rotor systems and the design solutions to meet such requirements are well defined and documented, the requirements for a VDR system are still evolving. Hence, the design of such components as the rotor blade assembly, and particularly, the torque tube/spar assembly, present unique challenges which heretofore have not been addressed. For example, the telescoping feature of the VDR blade assembly necessitates that the torque tube member be relatively small, i.e., in chord and thickness dimension, to accept the enveloping outer blade segment. Accordingly, the torque tube member must be particularly robust or stiff to carry the combined flapwise, edgewise and torsional loads imposed by the outer blade segment.
Furthermore, the frequency response, and accordingly, the stiffness requirements of the blade assembly will vary depending upon the mass or weight distribution of the VDR blade assembly. For example, as weight penalties are incurred in the outer blade segment by, for instance, the addition of counterweights, non-optimum blade design or inefficient manufacturing methods, the in-plane or edgewise frequency of the blade assembly will decrease proportionally. Insofar as it is desirable for a VDR system to be "stiff-in-plane", e.g., have an edgewise frequency greater than about 1.3 cycles per revolution, to avoid resonant instabilities when retracting the outer blade segment, weight penalties incurred in the outer blade segment must be counteracted by increasing the structural stiffness, and, accordingly, the weight of the torque tube member. Weight penalties occurring farther outboard, e.g., at the tip end of the outer blade segment, exacerbate the stiffness/weight penalties incurred in the torque tube member.
Inefficient weight distribution of the VDR blade assembly can also have adverse effects on the overall weight and complexity of the retraction/extension mechanism. For example, one kilogram (2.2 lbs) of additional mass at the tip end of the outer blade segment produces nearly 6000N (1,350 lbs) of centrifugal force when the VDR blade assembly is in a fully-extended position. Insofar as centrifugal load is transferred to the rotor hub assembly by the retraction/extension mechanism, the strength and, consequently, weight thereof will increase substantially to react the additional centrifugal load.
Fradenburgh et al. U.S. Pat. No. 3,71 3,751 addresses these problems by reducing the weight of the outer blade segment through the use of a modified structural spar and trailing edge pockets. More specifically, the structural spar of the outer blade segment includes a metallic C-shaped leading edge portion and a composite trailing edge pocket which is bonded thereto for closing the spar aft of the quarter chord. The modified spar and structural pockets eliminate certain requirements for leading edge counterweights thereby reducing the overall weight of the outer blade segment. While the teachings of Fradenburgh reduce the weight of the outer blade segment, the structural bonds formed between the C-shaped leading edge portion and the trailing edge pockets are potential sources of fatigue failure. Furthermore, the structural requirements for reacting the large compressive buckling loads are highest at the tip end of the outer blade segment. Accordingly, the configuration disclosed in Fradenburgh produces high tip end weight to accommodate the compressive buckling loads. As discussed supra, the concentration of weight at the tip end produces weight penalties in the torque tube, i.e., due to the edgewise stiffness requirements, and in the retraction/extension mechanism due to the imposition of large centrifugal loads.
The prior art references, including the Fradenburgh '751 patent, disclose constant wall thickness torque tube members which produce uniform weight distribution from the root end to the tip end thereof. Insofar as the imposed flapwise and edgewise bending loads increase from the root to the tip end of the torque tube member, it will be apparent that a constant wall thickness torque tube member which is optimally sized in the root end region for reacting such loads is non-optimally sized for the reduced loads experienced at the intermediate and tip end regions. Accordingly, a weight penalty is incurred in the regions outboard of the root end.